The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Carbohydrate polymers, cross-linked with various ions, such as boron, zirconium, and titanium, are used as high-viscous fracturing fluids in the oil and gas industry. Polysaccharides, such as guar and guar derivatives, are commonly used as viscosifying water-based fluids for fracturing treatments and for proppant transport. The proppant remains in the produced fracture in order to keep the fracture open and create a conductive channel extending from the well bore into the formation along the fracture length. After the fracture is complete, the recovery of the fracturing fluid is crucial to accelerate hydrocarbon production through the formed channel.
The recovery of the fracturing fluid is achieved by reducing the viscosity of the fluid such that the fluid flows naturally through the proppant pack. Chemical reagents, such as oxidizers, acids and enzymes are typically employed to break the polymer networks to reduce their viscosity. These materials are commonly referred to as “breakers.”
The timing of the breaking is important. Gels broken prematurely can cause proppant to settle out of the fluid before reaching a sufficient distance into the produced fracture and result in a premature screen-out. Premature breaking can also result in less desirable fracture width in the created fracture. On the other hand, too much delay in breaking the gel is not desirable either. Delayed breaking can cause significant setback in the hydrocarbon production. These factors, including reactivity levels versus temperature, delay mechanisms, and insufficient cleanup of the proppant pack impose significant complexity in designing a successful breaker system.
Ammonium persulfate or APS is one of the most widely used breakers in the industry. When ammonium persulfate is used, free sulfate radicals are generated due to thermal decomposition of the persulfate ions upon homolytic cleavage of the peroxo (O—O) bond. This free radical initiates a chain scission process by interacting with the polymer chain to abstract hydrogen, which results in the primary bond cleavage of either the mannose or galactose groups. The generated radicals propagate the process, further breaking the polymer into lower molecular weight fragments. This continues until the termination of the reaction occurs, mostly due to the combination of two radicals.
Although this process is very slow below 120° F. (50° C.), it becomes very rapid above this temperature, especially above 175° F. (80° C.). At such high temperatures, the ammonium persulfate breaker decomposes too quickly to be effective for use as a breaker in most applications.
Accordingly, a need exists for a breaker system that overcomes such limitations, and with higher control over a wide temperature range.